Method for stamping sheet metal



Patented 29, 1942 UNITED STA'lES PATENT OFFICE 2,306,861 METHOD roa s'rmmc suns-r METAL William Waite Broughton, Hoholms, N. 1., assignor to,National Lead Company,

New York, N. Y

acorporatlon of New Jersey No Drawing. Application Noyember 2, 1940, Serial No. 364,060

'5 Claims. (01. 76-4) This invention relates to the art of stamping ridges produced. in the parts, was carried out substantially according to I the following procedure:

There were first prepared from wood, plaster or other suitable material oversized mold patterns for both the male and female dies. From these oversize patterns sand molds were prepared and into these molds molten metal, usually a ferrous metal. e. g. steel, was poured to obtain when the mold was removed, after cooling, rough oversized castings. These rough oversized castings required trimming and finishing to proper dimensions. For this purpose a three-dimensional duplicating machine was employed. Such machines employ a guide pattern made of some suitable wood, e. g. mahogany, which is shaped to conform as exactly as possible to the predetermined shape of the finished die. In operation. the duplicating machine following the contours of the guide pattern, cuts down the rough casting but, since the cutting blade makes each traverse in a new plane, the casting as it comes from the machine is ridged and these ridges have to be trimmed off and the die finished by hand. In this manner both the male and female dies are formed. The dies after trimming and finishing by hand are fitted into a stamping press and a trial stamping made in order to determine spring-back eil'ect. "Spring-back" may be defined as the difference in shape between th actual contours of dies and the shape of the stamping produced in them. It is always present provided that the material to be stamped is elastic. Certain variables, including the physical properties and chemical composition of the sheet metal, the shape of the dies, the pressure exerted, etc., all influence "spring-back and its extent can not be positively determined, a priori, when de signing the dies. Although effort is made in the patterns for the Keller machine to compensate for spring-back" the production of a finished set of dies which require no correction for sprin back is to be regarded as a fortunate accident. Hence, it is most frequently necessary to correct a set of dies in order to compensate for back after a trial stamping has been made with the new set of dies. After determining the extent of th spring-back the dies are reshaped to compensate for the spring-back. This reshaping is most often done by preparing a new or reworking the original guide pattern for the Keller machine and recutting the die accordingly, after which a second trimming down of the i-shing by hand is necessary.

In the automotiveindustry the dies which are first formed are often used to stamp out parts for the yearly dealers models. If time be taken in every instance to correct for spring-back it would often be impossible to assemble suficient automobiles in time for the fixed date for the annual first showing of new models. On the other hand, if parts are stamped from dies which are not corrected iorspring-back" the various parts do not readily fit much pressing, hammering and forcing: before the automobile is finally" assembled.

It will be at once appreciated that the pro-'- cedure just above described is costly, .laborious,

and time consuming. Tit-involves certain readily apparent disadvantagesi- First after serving its function in the duplicating machine themahog any-guide 'patternhas no further usefulness.

Second, the correction of steel dies for spring-.

back is a tedious and expensive task due to the necessity for actually machining the steel. Third. there is always present in the automotive industry the dilemma that the yearly model will not be assembled on time or that the stamped parts will not fit properly together. The present invention provides a simple and economical means for overcoming these and other disadvantages inherent in prior art methods of preparing dies and in their use.

Thus, it is one object of the present invention to provide a method for preparing guide patterns which may be employed in a three-dimensional duplicating machine and which are, themselves,

useful as dies. It is another object of the present further object of the invention to provide a new technique for stamping sheet metal parts. These Kellermachine and fintogetherv and there is In its broadest aspect the invention also contemplates a stamping method including the following steps:

(a) The formation of mold patterns from wood or plaster or other suitable material;

(b) The preparation of molds, usually molds, from the mold patterns;

(0) The casting in such molds of a rough, oversized ferrous metal casting and a nonferrous alloy casting.

With respect to the foregoing three steps it is to be noted that according to the invention it is preferable that different mold patterns and different molds be made for the ferrous metal and the non-ferrous alloy castings. This is so because the non-ferrous alloy can be cast to more exact contours than the ferrous metal. Thus, a plaster pattern made very nearly exactly to the desired shape of the die, with due allowance for solidification shrinkage may be used for preparing the mold for the non-ferrous alloy casting while a rough, oversized wooden pattern made for the mold for the ferrous metal casting.

(d) The shaping and trimming of the non-ferrous alloy casting to the predetermined exact shape of the die;

(e) The carrying out of a trial stamping with the shaped and trimmed alloy casting in order to determine spring-back" effect;

if) The correction of th shaped and trimmed alloy casting to compensate for springback."

With respect to the foregoing last mentioned three steps, it will be appreciated that if the mold pattern and mold for the alloy casting is prepared carefully enough and the proper alloy selected, as herein-after suggested, the step d may be eliminated and a trial stamping made directly with the cast alloy dies. Furthermore, as to step I the correction may be made in any suitable manner. For instance, the mold pattern can be re-worked,

sand

a new sand mold made, the alloy casting melted and recast in the new mold. Alternatively, if very little correction be required the alloy casting can be corrected by working directly on the casting.

(9) The use of the corrected cast alloy die in a three-dimensional duplicating machine as a pattern for cutting down the rough, oversized ferrous metal casting.

(h) The further use of the corrected cast alloy die for forming the initial requirements of stamped metal parts.

parts may be made while the ferrous metal casting is being trimmed of the ridges produced by the Keller machine and finished by hand.

(i) The melting of the cast alloy die and the reuse of the metal.

Non-ferrous metal alloys which are useful in the practice of the present invention are those which possess (1) good molding characteristics,

i. e. are easily melted and poured without appreciable drossing or change in composition, readily assume the contours of the mold and tend, when cool, to shrink away from the mold while retaining the sharp outlines of the mold; (2) easy workability, i. e. may be readily trimmed with cutting tools, files and the like; (3) suiiicient hardness, tensile, impact and compressive strength to permit their use in forming dies which may be used for stamping sheet metal. For instance, alloys may be used in the practice of the present invention which possess physical properties of the following order:

Tensile strength Above 30,000 P. S. I. Impact strength Not substantially below 4 ft.-lbs. to break bar /4 1. sq. Compressive strength Above,50,000 P. S. I. Brinell hardness Above (10 mm. ball, 500 kg. load,

30 sec.)

One class of alloys which possesses the above listed characteristics and properties are zinc alloys in which there may be present as alloying ingredients minor amounts of aluminum, copper and/or magnesium. For instance, zinc alloys which are especially suited for the practice of the present invention will have a composition of the following order:

Aluminum Between about 2 percent and about 10.0 percent, preferably about 4 percent Copper Between about 0 percent and about 5.0 percent, preferably about 3.0 percent Zinc Balance the stamped sheet metal parts to be assembled in an automobile model, oversized wood patterns and nearly exact plaster patterns are made according to previously prepared designs. From the oversized patterns sand molds for ferrous metal castings are prepared and from the plaster patterns molds for alloy castings. Ferrous metal castings are made which may be about /2 inch oversized. The alloy castings are made using an alloy consisting of about 4.0% aluminum, 3.25% copper, 0.04% magnesium and the balance zinc. Such a casting will break cleanly from the mold and will retain the sharp contours thereof so that if the pattern and the mold are carefully made and the casting skillfully carried out the said casting, after cleaning, may be fitted to a press and a trial stamping made in order to determine the spring-back. The plaster patterns now are reworked in order to produce molds from which may be cast alloy dies corrected for spring-back. The corrected alloy dies are now employed as guide patterns in a Keller duplicating machine in the cutting down of the rough, oversized ferrous metal castings. While the ferrous metal castingsafter kellering are being finished by hand or at any other convenient time the alloy dies are employed to stamp out the initial requirements of stamped metal parts.

From the foregoing it will be seen that the uncertainties of using in a duplicating machine a pattern which is not corrected for spring-back are entirely overcome and the possibility of a tedious and expensive correction of the ferrous metal dies entirely eliminated. Furthermore, the use as a pattern in the duplicating machine of a die which can be used for turning out stamped articles makes available additional sets of useful dies. In addition, by utilizing a metal alloy which can be melted and re-used there results a considerable saving over the prior art procedure.

I claim:

1. Method of preparing ferrous metal dies requiring no correction for spring-back and used for stamping sheet metal which comprises casting according to a predetermined die design a rough, oversized ferrous metal die and a nonferrous alloy die conforming more exactly to said design, said non-ferrous alloy die having suflicient hardness to permit the use of said die for the stamping of sheet metal, making a trial stamping with said non-ferrous alloy die, determining upon said trial stamping the extent of "spring-back effect and correcting said non-ferrous alloy die therefor, and subsequently employing said non-ferrous alloy die in a three-dimensonal duplicating machine as a guide pattern for cutting down the rough oversized ferrous metal die.

2. Method of preparing ferrous metal dies requiring no correction for spring-back and used for stamping sheet metal which comprises casting according to a predetermined die design a rough, oversized ferrous metal die and a zinc alloy die conforming more exactly to said design, said zinc alloy die having. sufficient hardness to permit the use of said die for the stamping of sheet metal, making a trial stamping with said zinc alloy die, determining upon said trial stamping the extent of spring-back effect and correcting said zinc alloy die therefor, and subsequently employing said zinc alloy die in a three-dimensional duplicating machine as a guide pattern for cutting down the rough, oversized ferrous metal die.

3. Method of preparing ferrous metal dies requiring no correction for spring-bac and used for stamping sheet metal which comprises casting in molds prepared according to a predetermined die design a rough, oversized ferrous metal die and a zinc alloy die conforming more exactly to said design, determining the extent of springback effect by performing a trial stamping with said zinc alloy die, melting said alloy die and recasting it in a mold conforming to the die design and also corrected for spring-back and subsequently employing said newly-cast zinc alloy die in a three-dimensional duplicating machine as a guide pattern for cutting down the rough, oversized ferrous metal die.

4. Method of preparing ferrous metal dies requiring no correction for spring-back and used for stamping sheet metal which comprises casting according to a predetermined di design a rough, oversized ferrous metal die and a zinc alloy die conforming more exactly to said design, said zinc alloy die having a tensile strength above 30,000 pounds per square inch, an impact strength not substantially below 4 foot-pounds required to fracture a bar thereof A inch square, 9. compressive strength above 50,000 pounds per square inch and a Brinell hardness above 80, making a trial stamping with said zinc alloy die, determining upon said trial stamping the extent of springback effect and correcting said zinc alloy die therefor and subsequently employing said zinc alloy die in a three-dimensional duplicating machine as a guide pattern for cutting down the rough, oversized ferrous metal die.

. 5. Method of preparing ferrous metal dies requiring no correction for spring-back and used for stamping sheet metal which comprises casting according to a. predetermined die design a rough, oversized ferrous metal die and a zinc alloy die conforming more exactly to said design, a zinc alloy consisting of between about 2.0 percent and about 10.0 percent aluminum; between zero percent and about 5.0 percent copper; magnesium between zero percent and about 0.3 percent, balance zinc, making a trial stamping with said zinc alloy die, determining upon said trial stamping the extent of spring-back effect and correcting said zinc alloy die therefor and subsequently emplcying said zinc alloy die in a three-dimensional duplicating machine as a guide pattern for cutting down the rough, oversized ferrous metal die.

WILLIAM WAITE BROUGHTON. 

